[google][compute] Update to API v1

Simplify attachment of boot disk, get_as_boot_disk no longer needed
Add missing Disk.get_object method
Allow setting deviceName when attaching a disk
Add disks mocks
[google] Handle 500 errors from GCE
[google][compute] Add support for blank disks (i.e. remove code that required only image or snapshot based disks to be created)